1. Field of the Invention
The present invention generally relates to a video projector such as a projection type television apparatus and, more particularly, to a back projection screen used therein for the reproduction of projected images thereon.
2. Description of the Prior Art
The projection type television apparatus employing a projection screen which are currently available in the market can be generally classified in two types depending on the direction in which image is projected onto the screen. The two types are a front projection type and a back projection type. As a matter of course, the front projection screen is reflective and operates to reflect the imagewise rays of light that are projected from one or three cathode ray tubes, towards viewers in front of the television apparatus. The back projection screen is light-transmissive and operates to pass the imagewise rays of light therethrough towards viewers in front of the television apparatus.
Of these two types of projection screens, the present invention pertains to the back projection type.
A composite projection screen that is used in the back projection type is well known to generally include a plastic sheet or plate having a Fresnel lens embossed or formed in any suitable manner on one surface thereof, and a light-diffusing sheet or plate held or integrated in a face-to-face relationship with the plastic sheet. An example of the back projection type television apparatus utilizing the composite projection screen is illustrated in a schematic side sectional representation of FIG. 3. Reference to FIG. 3 will be made for the discussion of the manner in which the composite back projection screen is supported according to the prior art.
The television apparatus comprises a generally rectangular upright cabinet 1 including a front wall 1b having a rectangular front opening 1a defined therein, a correspondingly rectangular composite screen Sa secured to the front wall 1b which closes the front opening 1a, and a rear wall 1c having defined therein a generally rectangular rear opening which is closed by a rear covering 6. The cabinet 1 accommodates therein a projector unit 4 which may comprise one or three projection cathode ray tubes and is positioned in a lower region of the interior of the cabinet 1 for directing imagewise rays of light towards a reflector 5 secured to the rear covering 6 within the cabinet 1. The reflector 5 is supported and inclined at an angle so that the imagewise rays of light from the projector unit 4 can be reflected towards the composite screen Sa.
The composite screen Sa used therein comprises a laminar structure including a first plate 2 made of acrylic resin and having a Fresnel lens formed on one surface thereof (plate 2 is hereinafter referred to as a "Fresnel lens plate") and a second plate 3 made of the same acrylic resin as the first plate 2 and having a lenticular lens formed on one surface thereof (plate 3 is hereinafter referred to as a "lenticular lens plate").
The composite screen Sa is mounted on the cabinet 1 in a manner as best shown in FIGS. 4 and 5. More specifically, the Fresnel lens plate 2 and the lenticular lens plate 3 have an equal height and an equal width as viewed from the position of a viewer in front of the television apparatus. However, the lenticular lens plate 3 has a thickness smaller than the thickness of the Fresnel lens plate 2. The Fresnel lens plate 2 and the lenticular lens plate 3 are combined together in a face-to-face relationship by the use of double-sided adhesive tape, generally identified by 7, which are applied to four side edge portions of the Fresnel lens plate 2 and the lenticular lens plate 3. The assembly of the Fresnel and lenticular lens plates 2 and 3 is secured to the front wall 1b of the cabinet 1 by four elongated fixtures 8 and set screws 9 so that the front opening 1a is covered with the Fresnel and lenticular lens plates 2 and 3 disposed close towards but remote from a projection source including the projector unit 4 and the reflector 5.
According to the prior art support system, as best shown in FIG. 5, the bottom edge of the composite screen Sa is held in contact with the bottom fixture 8 by gravitational force, and the design has been made so that a clearance is formed between the top side of the composite screen Sa and the top fixture 8 and also between any one of the opposite side edges of the composite screen Sa and the associated side fixture 8. The purpose of the clearance is for accommodating any possible thermal expansion of any one of the Fresnel and lenticular lens plates 2 and 3.
The acrylic resin used for any one of the Fresnel and lenticular lens plates 2 and 3 has been found to exhibit the physical characteristics as shown in FIGS. 6(a), 6(b) and 6(c). FIG. 6(a) illustrates the change in coefficient of linear expansion with respect to change in temperature, FIG. 6(b) illustrates the change in moisture content with respect to changes in relative humidity, and FIG. 6(c) illustrates the respect to changes in size with change in moisture content.
Although the Fresnel and lenticular lens plates 2 and 3 have different thicknesses, the amount of change in size resulting from an increase in relative humidity which in turn brings about an increase of the moisture content attains a level substantially equal to the level exhibited when the moisture content is in a saturated condition. However, during a period in which the moisture content varies from an initial level to a saturated level, the extent to which the size of the acrylic resin forming the Fresnel lens plates 2 changes is different from the extent in forming the lenticular elns plate 3 in the following manner because of the difference in thickness between the Fresnel and lenticular lens plates 2 and 3.
In the prior art composite screen Sa, since the lenticular lens plate 3 is greater in thickeness than the Fresnel lens plate 2 and is therefore smaller in volume than the Fesnel lens plate 2, the moisture content of the lenticular lens plate 3 is apt to be quickly saturated. Moreover, since the lenticular lens plate 3 is exposed to the outside of the cabinet 1, the lenticular lens plate 3 is susceptible to absorption of ambient moisture. Because of this, the lenticular lens plate 3 tends to undergo more considerable expansion and deformation than in the Fresnel lens plate 2.
On the other hand, in the prior art support system all four sides of the lenticular lens plate 3 are connected with the associated sides of the Fresnel lens plate 2 by double-sided adhesive tape 7 applied thereto and the bottom edge of the lenticular lens plate 3 is clamped against the front wall 1b by the bottom fixture 8 together with the bottom edge of the Fresnel lens plate 2. Accordingly, an abrupt expansion of the lenticular lens plate 3 tends to be constrained which results in the tendency of the lenticular lens plate 3 to undulate. It is also found that, when the expansion of the lenticular lens plate 3 is constrained, swelling occurs at a lower region of the compsoite screen Sa under the influence of the weight of the lenticular lens plate itself and a void is consequently formed between the Fresnel and lenticular lens plates 2 and 3.
The undulation of the lenticular lens plate 3 and the formation of the void at the lower region of the composite screen Sa cause a considerable reduction in screen performance. Deformation is particularly considerable where the composite screen Sa is made larger in size with the increase of the demand for the projection type television apparatus utilizing the relatively large screen. Thereby the image projected on the relatively large composite screen Sa is apt to be distorted once the lenticular lens plate is deformed.
The Japanese Laid-open Patent Publication No. 60-61738 published April 9, 1985, discloses a technique similar to that discussed hereinabove with reference to FIGS. 3 to 5, but this technique differs therefrom because double-sided adhesive tape is applied to the top and side edge portions of the Fresnel and lenticular lens plates with the bottom portions thereof kept free. Even the technique disclosed in this publication is insufficient and the occurrence of the undulation and swelling in the composite screen cannot be eliminated.